I would suggest setting up at the very least a good chore plan, or other objective if you do get him a data plan. At 15 he should be coming up with an educational goal, or career goal, or some shorter term goal. Does he know what he wants to be when he grows up? Does he know the educational requirements for reaching that goal?
I still say in most areas getting a job at 15 is near impossible unless you know someone and get him hired there. It is not too young however to start doing some community volunteer work, and honestly might help him build some reputation, higher up universities actually expect to see charitable work, group activities such as sports, or club involvement on your application.
These are all things you should be able to expect for your extra data plan bill. Pretty soon it's going to turn to the issue of the first car, and gas, and insurance.
I say if he is on a good path towards reaching goals then it is worth helping them out on the way. If they are on the wrong path <like I always was> Then the help they need isn't data plans, it's direction, and the distractions of Facebook and such won't help.